Process for fabricating micromachine

ABSTRACT

A method for manufacturing a micromachine is provided which can remove a sacrifice layer and can perform sealing without using a specific packaging technique. In a method for manufacturing a micromachine ( 1 ) including an oscillator ( 4 ), a step of forming a sacrifice layer around a movable portion of the oscillator ( 4 ); a step of covering a sacrifice layer with an overcoat film ( 8 ), followed by the formation of a penetrating hole ( 10 ) reaching the sacrifice layer in the overcoat layer ( 8 ); a step of performing sacrifice-layer etching for removing the sacrifice layer using the penetrating hole ( 10 ) in order to form a space around the movable portion; and a step of performing a film-formation treatment at a reduced pressure following the sacrifice-layer etching so as to seal the penetrating hole ( 10 ).

TECHNICAL FIELD

The present invention relates to a method for manufacturing amicromachine including an oscillator.

BACKGROUND ART

In recent years, concomitant with the advancement of techniques formanufacturing microstructures on substrates, so-called micromachines(Micro Electro-Mechanical Systems, hereinafter referred to as “MEMS”)and compact apparatuses incorporating MEMSs have drawn attentions. TheMEMS is a device composed of an oscillator, which is a movablestructural element, and a semiconductor integrated circuit or the likewhich controls the drive of the oscillator and which is electrically andmechanically coupled therewith. In addition, the oscillator isincorporated in part of the device, and the drive of the oscillator iselectrically performed using the Coulomb force or the like betweenelectrodes.

Of the MEMSs as described above, in particular, devices formed using asemiconductor process have the following various features. That is, forexample, the devices each require a small area, can realize a high Qvalue (quality indicating the sharpness of resonance of an oscillationsystem), and can be integrated with another semiconductor device(integration); hence, the use as a high-frequency filter for wirelesscommunication has been proposed (for example, see C. T.-C. Nguyen,“Micromechanical components for miniaturized low-power communications(invited plenary),” proceedings 1999 IEEE MTT-S International MicrowaveSymposium RF MEMS Workshop, Jun. 18, 1999, pp. 48-77).

Incidentally, when a MEMS is integrated with another semiconductordevice, the structure has been proposed in which encapsulation isperformed for the oscillator, which is a part of the MEMS, so that awiring layer or the like is further provided above the oscillator (forexample, see Japanese Unexamined Patent Application Publication No.2002-94328 (p. 7 and FIG. 10)). However, when the oscillator isencapsulated, a hollow structure must be formed around the oscillator,that is, it is required that a space around a movable portion of theoscillator is ensured so as to place the oscillator in a movable state.For ensuring the space around the movable portion described above, ingeneral, so-called sacrifice-layer etching is performed.

The sacrifice-layer etching is etching in which a thin film is formedbeforehand around the movable portion of the oscillator and is thenremoved by etching so as to form the space (gap) around the movableportion. In addition, the thin film formed around the movable portionfor sacrifice-layer etching is called a sacrifice layer.

However, the integration of the MEMS with another semiconductor devicehas various problems. In general, for this integration, a process formanufacturing the MEMS (particularly, the oscillator thereof) isperformed in a final step which is added to a manufacturing process(such as a CMOS process) of said another semiconductor. Accordingly, inthe process for manufacturing the MEMS, in order to avoid adverseinfluences on the semiconductor device which is already formed, ahigh-temperature process cannot be performed. That is, the oscillatormust be formed at a low temperature, and as a result, the processtherefor may not be easily performed in some cases.

On the other hand, when the part of the MEMS, that is, the oscillator,is encapsulated, a wiring layer or the like may be further formedthereabove, and as a result, even though the oscillator is formed at ahigh temperature, an adverse influence of the high-temperature processon the wiring layer or the like can be avoided. However, in the casedescribed above, since the space around the movable portion of theoscillator formed by sacrifice-layer etching is vacuum-sealed, aspecific packaging technique using an insulating material or the like isrequired (for example, see Japanese Unexamined Patent ApplicationPublication No. 2002-94328 (p. 7 and FIG. 10). That is, since apackaging step for vacuum sealing is required, manufacturing cannot beeasily performed using the existing semiconductor process (such as aCMOS process), and as a result, the production efficiency of a deviceincluding the MEMS may be decreased in some cases.

Accordingly, an object of the present invention is to provided a methodfor manufacturing a micromachine, in which the oscillator, which is thepart of the MEMS, is sealed using sacrifice-layer etching in order toachieve easier formation of the MEMS, and in which even in the casedescribed above, removal of a sacrifice layer and sealing can beperformed without using any specific packaging technique.

DISCLOSURE OF INVENTION

The present invention provides a method for manufacturing a MEMSincluding an oscillator in order to achieve the object described above.The method described above comprises a step of forming a sacrifice layeraround a movable portion of the oscillator; a step of covering asacrifice layer with an overcoat film, followed by the formation of apenetrating hole in the overcoat layer which reaches the sacrificelayer; a step of performing sacrifice-layer etching for removing thesacrifice layer using the penetrating hole in order to form a spacearound the movable portion; and a step of performing a film-formationtreatment at a reduced pressure so as to seal the penetrating holefollowing the sacrifice-layer etching.

According to the manufacturing method of a MEMS, which has the proceduredescribed above, since the step of forming a sacrifice layer, the stepof covering the sacrifice layer with an overcoat film, and the step ofperforming sacrifice-layer etching are performed, a wiring layer or thelike can be further formed above the overcoat film. That is, after thesteps described above, a step of forming a wiring layer or the like maybe performed. Hence, when the oscillator is formed in a previous step(such as a step prior to an aluminum step of a CMOS process), althoughthe oscillator is formed at a high temperature, the high temperatureprocess will not adversely influence on a wiring layer or the like.

In addition, since the step of performing a film-formation treatment ata reduced pressure is performed so as to seal the penetrating hole, inthis step, the space around the movable portion of the oscillator issealed in an evacuated state. Furthermore, since the penetrating hole issealed in the film-formation treatment at a reduced pressure, afilm-formation technique of a semiconductor process (such as a CMOSprocess) can be used as it is. Accordingly, the film-formation step canbe continuously performed in conjunction with other steps of thesemiconductor process, and in addition, a specific packaging techniquefor vacuum sealing is not required.

According to the method for manufacturing a MEMS of the presentinvention, since the oscillator, which is the part of the MEMS, issealed, even when the oscillator is formed at a high temperature,adverse influence thereof on a wiring layer or the like can be avoided,and as a result, easier formation of a MEMS can be achieved.Furthermore, since the sealing of the space formed by thesacrifice-layer etching is performed by the film-formation treatment ata reduced pressure, without using any specific packaging technique, theremoval of the sacrifice layer and the sealing can be performed. Hence,according to the present invention, even when a MEMS is integrated withanother semiconductor device, the production efficiency of the deviceincluding the MEMS can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views illustrating one example of the structure of aMEMS obtained by the present invention; FIG. 1A is a plan view; FIG. 1Bis a front view; and FIG. 1C is a cross-sectional view taken along A-A′in FIG. 1A.

FIGS. 2A to 2D are views (part 1) illustrating one procedure of a methodfor manufacturing a MEMS according to the present invention, and FIGS.2A to 2D are showing respective steps.

FIGS. 3A to 3D are views (part 2) illustrating one procedure of a methodfor manufacturing a MEMS according to the present invention, and FIGS.3A to 3D are showing respective steps.

FIG. 4 is a view illustrating another example of the structure of a MEMSobtained by the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, with reference to figures, a method for manufacturing aMEMS according to the present invention will be described. However, itis to be naturally understood that the following embodiments which areto be described are merely preferable detailed examples of the presentinvention, and that the present invention is not limited thereto.

Prior to the description of the method for manufacturing a MEMS, aschematic structure of the MEMS will be described. In this embodiment, aMEMS used as a high-frequency filter for wireless communication will bedescribed by way of example. FIGS. 1A to 1C are views illustrating oneexample of the structure of a MEMS obtained by the present invention.

As shown in FIG. 1A, in addition to an input electrode 2 and an outputelectrode 3, a MEMS 1 which will be described has a belt-shaped beamtype oscillator (hereinafter simply referred to as “oscillator”) 4formed of a conductive material such as polycrystalline silicon(Poly-Si) containing phosphorus. When a voltage having a specificfrequency is applied to the input electrode 2, a beam portion (movableportion) of the oscillator 4 oscillates at its natural oscillationfrequency, and the capacitance of a capacitor, which is formed of aspace between the output electrode 3 and the movable portion of theoscillator 4, is changed and is then output from the output electrode 3.Hence, when the MEMS 1 is used as a high-frequency filter, compared to ahigh-frequency filter using a surface acoustic wave (SAW) or a thin filmacoustic wave (FBAR), a high Q vale can be realized.

The input electrode 2, the output electrode 3, and the oscillator 4,which constitute the MEMS 1, are all formed as shown in FIG. 1C over thestructure composed, for example, of a SiN (silicon nitride) film 7, aSiO₂ film 6, and a semiconductor substrate (hereinafter referred to as“Si substrate”) 5 formed of Si (single crystal silicon), provided inthat order from the top side. Since being formed over the Si substrate5, the MEMS 1 can be integrated with another semiconductor device.

In the MEMS 1, since the movable portion of the oscillator 4 oscillatesat the natural oscillation frequency thereof, the space around themovable portion of the oscillator 4 is ensured. However, since themovable portion of the oscillator 4 is covered with an overcoat film 8as described later, the space is ensured in the vertical and thehorizontal directions of the cross-section of the movable portion, thatis, is ensured all around the cross-section thereof.

In addition, at the upper side of the oscillator 4, in order to seal themovable portion of the oscillator 4 by covering, the overcoat film 8composed, for example, of a SiN film is formed. By the presence of thisovercoat film 8, in the MEMS 1, while being placed in a movable state,the oscillator 4 is sealed, and as a result, a wiring layer or the likecan also be further provided above the overcoat film 8. By thisstructure, it is said that the MEMS 1 is a device which can be suitablyintegrated with another semiconductor device.

In this embodiment, on part of the upper surface of the overcoat film 8,a sputter film 9 composed, for example, of an Al—Cu (aluminum-copper)film or an Al—Si (aluminum-silicon) film is formed. This sputter film 9is provided for sealing the penetrating holes 10 which are provided inthe overcoat film 8 for sacrifice-layer etching in order to ensure thespace around the movable portion of the oscillator 4.

Next, a method for manufacturing the MEMS 1 described above, that is, amethod for manufacturing a MEMS, according to the present invention,will be described. FIGS. 2A to 3D are views illustrating one procedureof a method for manufacturing a MEMS according to the present invention.

When the MEMS 1 having the structure described above is formed, first,as shown in FIG. 2A, the SiO₂ film 6 and the SiN film 7, eachfunctioning as an insulating film, are formed on the Si substrate 5 by areduced pressure CVD (chemical vapor deposition) method. In addition, onthe surface of the above film, as shown in FIG. 2B, a film is formedusing a selectively etchable material such as polycrystalline silicon(Poly-Si) containing phosphorus (P), and subsequently, by using knownlithographic and dry etching techniques, a lower wire 11 is formed bypatterning.

After the lower wire 11 is formed by patterning, as shown in FIG. 2C, aSiO₂ film is formed, for example, by a reduced pressure CVD method andis then patterned by known lithographic and dry etching techniques, sothat the lower wire 11 is covered with a SiO₂ film 12. This SiO₂ film 12functions as a sacrifice layer as described later.

Next, as shown in FIG. 2D, on the SiO₂ film 12, a Poly-Si film isformed, for example, by a reduced pressure CVD method and is thenpatterned by using known lithographic and dry etching techniques, sothat the belt-shaped oscillator 4 made of Poly Si is formed.

After the oscillator 4 is formed, subsequently, as shown in FIG. 3A, aSiO₂ film is formed, for example, by a reduced pressure CVD method andis then patterned by using known lithographic and dry etchingtechniques, so that the oscillator 4 is covered with a SiO₂ film 13.This SiO₂ film 13 also functions as a sacrifice layer. Accordingly, theperiphery of the movable portion of the oscillator 4, that is, all thesurfaces including side surfaces of the cross section of the oscillator4 in the vertical and horizontal directions are covered with the SiO₂film 12 and the SiO₂ film 13. That is, in the lower direction of thecross section of the oscillator 4, the SiO₂ film 12 is present, and inthe horizontal direction and the upper direction of the cross section ofthe oscillator 4, the SiO₂ film 13 is present.

As described above, after the SiO₂ film 12 and the SiO₂ film 13, eachfunctioning as a sacrifice layer, are formed, as shown in FIG. 3B, onthe surfaces thereof, a SiN film 14 is formed, for example, by a reducedpressure CVD method. This SiN film 14 functions as an overcoat filmcovering the sacrifice layers. In addition, in this SiN film 14, thepenetrating holes 10 reaching the sacrifice layer (one of the SiO₂ film12 and the SiO₂ film 13) are formed by known lithographic and dryetching techniques.

After forming the penetrating holes 10, sacrifice-layer etching forremoving the sacrifice layers is performed using the penetrating holes10, so that the space is formed around the movable portion of theoscillator 4. That is, as shown in FIG. 3C, using a solution, whichselectively removes SiO₂, such as an aqueous hydrofluoric acid solution(DHF solution), the SiO₂ film 12 and the SiO₂ film 13 are removed.Accordingly, around the movable portion of the oscillator 4, that is,along the entire periphery of the cross-section of the movable portion,a space (gap) corresponding to the thickness of the sacrifice layer isformed, and hence the movable portion of the oscillator 4 is allowed tooscillate at the natural oscillation frequency.

After the sacrifice-layer etching, a film-formation treatment which isthe most characteristic step in this embodiment is performed at areduced pressure. In particular, for example, a film-formation treatmentby sputtering is performed in an evacuated state, and as shown in FIG.3D, the sputter film 9 is formed for sealing the penetrating holes 10.Since the film-formation treatment is executed by sputtering, as areactant gas used in this step, for example, an argon (Ar) gas, which isan inert gas, may be mentioned. In addition, as the sputter film 9, forexample, a thin film of a metal or a metal compound, such as an Al—Cufilm or an Al—Si film, may be mentioned. Subsequently, after beingformed, the sputter film 9 is patterned into wires or the like by usingknown lithographic and dry etching techniques.

Through the procedure (respective steps) as described above, the MEMS 1shown in FIGS. 1A to 1C is formed. However, the manufacturing methodperformed by the above procedure is not only applied to the MEMS 1having the structure shown in FIGS. 1A to 1C but may also be applied toa MEMS having a different structure as long as the MEMS is formed bysacrifice-layer etching which is performed using a penetrating holeprovided in an overcoat film.

FIG. 4 is a view illustrating another example of the structural of aMEMS obtained by the present invention. Since the lower wire 11 isburied, a MEMS 1 a shown in the figure is different from the MEMS 1shown in FIGS. 1A to 1C; however, the MEMS 1 a described above can evenbe manufactured by a procedure (respective steps) similar to that forthe above MEMS 1. That is, it is considered that the penetrating holes10 provided in the overcoat film 8 for sacrifice-layer etching can besealed by a film-formation treatment using sputtering.

In addition, in both the MEMS 1 and the MEMS 1 a shown in FIGS. 1A to 1Cand 4, the case in which the oscillator 4 is a belt-shaped beam type isdescribed by way of example; however, even in the case of a so-calledring type oscillator or disc type oscillator, as long as a hollowstructure is used in which a space for a movable portion is ensuredtherearound, the manufacturing method according to above procedure canbe applied in the exactly same manner as that described above.Furthermore, as means for driving oscillation in the oscillator, thecase in which static electricity is used is described in the aboveexample; however, the means is not always limited to the electrostaticdrive, and for example, it may be applied to a piezoelectric driven FBARin the exactly same manner as described above.

As described above, according to the method for manufacturing a MEMSdescribed in this embodiment, since there are provided the steps offorming the SiO₂ film 12 and the SiO₂ film 13 functioning as a sacrificelayer around the oscillator 4, the step of covering the sacrifice layerswith the SiN film 14 which is the overcoat film, and the step ofperforming the sacrifice-layer etching, a wiring layer or the like canbe further disposed above this SiN film 14. That is, after the stepsdescribed above, a step of forming a wiring layer or the like can beperformed. Hence, by forming the oscillator 4 in a step prior to theabove step, the oscillator 4 can be formed at a position lower than thatof a metal wiring layer or the like. As a result, even when theoscillator 4 is formed at a high temperature, the high-temperatureprocess will not adversely influence the wiring layer or the like, andhence the easier formation of the oscillator 4 can be achieved.

Furthermore, according to the method for manufacturing a MEMS describedin this embodiment, since the step of sealing the penetrating holes 10by the film-formation using sputtering is performed following thesacrifice-layer etching, the space around the movable portion of theoscillator 4 can be sealed in the step described above. Hence, aspecific packaging technique using an insulating material or the like isnot required. That is, without performing a packaging step for a vacuumsealing, the space formed by the sacrifice-layer etching around themovable portion of the oscillator 4 can be sealed.

In addition, it may also be considered that the sputter film 9 used forsealing is to be used as wires or the like. That is, it may also beconsidered that by using the sputter film 9 for forming wires or thelike, the penetrating holes 10 are sealed; hence, in the case describedabove, the sealing and the formation of wires or the like are realizedin the same step, and as a result, the improvement in efficiency of themanufacturing process can be very effectively realized.

Furthermore, since the penetrating holes 10 are sealed by thefilm-formation treatment using sputtering, a film-formation technique ofa semiconductor process (such as a CMOS process) can be used as it isand, in addition, the technique described above can be continuouslyperformed in conjunction with other steps of the above semiconductorprocess. That is, sealing can be performed in a so-called in-lineprocess. As a result, integration with a CMOS process or the like can bevery easily performed, and in addition to that, MEMS evaluation in awafer state can also be performed.

As has thus been described, when a MEMS is formed by the manufacturingmethod described in this embodiment, even when the MEMS is integratedwith another semiconductor device, the manufacturing of the MEMS can beperformed in an existing semiconductor process (such as a CMOS process),and as a result, the production efficiency of the device including theMEMS can be improved.

In particular, as described in this embodiment, when being performed bythe film-formation treatment using sputtering, the sealing is executedin an argon gas which is an inert gas, and hence it is very preferablein terms of safety and reliability.

1. A method for manufacturing a micromachine including an oscillator,comprising: a step of forming a sacrifice layer around a movable portionof the oscillator; a step of covering the sacrifice layer with anovercoat film, followed by the formation of a penetrating hole reachingthe sacrifice layer in the overcoat layer; a step of performingsacrifice-layer etching for removing the sacrifice layer using thepenetrating hole in order to form a space around the movable portion;and a step of performing a film-formation treatment at a reducedpressure following the sacrifice-layer etching so as to seal thepenetrating hole.
 2. The method for manufacturing a micromachine,according to claim 1, wherein the method is applied to a micromachinehaving means for driving oscillation in the oscillator.
 3. The methodfor manufacturing a micromachine, according to claim 2, wherein staticelectricity is used as the means for driving oscillation.
 4. The methodfor manufacturing a micromachine, according to claim 2, whereinpiezoelectricity is used as the means for driving oscillation.
 5. Themethod for manufacturing a micromachine, according to claim 1, whereinthe film-formation treatment at a reduced pressure is a film-formationtreatment by sputtering.